Laser Quenching of Ionic Nitrided Steel: Effect of Process Parameters on Microstructure and Optimization
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NTRODUCTION
DUE to the rapid developments in the surface engineering field, conventional techniques for surface treatment like carburizing and flame hardening are being replaced by more local heating techniques, using advanced sources such as plasma, laser, and electron beams. Laser technologies are becoming an acknowledged means of surface hardening due to their ability to locally heat a very limited amount of surface, which is subsequently rapidly quenched by the surrounding colder zones. Cooling can occur so rapidly that crystallization can be inhibited to produce amorphous microstructures via rapid solidification.[1,2] In common hardening applications, a laser is used to rapidly heat a thin layer of the substrate (iron based) above the austenitizing temperature range, after which it is cooled at a very fast rate due to self-quenching by the conduction of heat into the bulk material, resulting in a martensitic structure.[3–5] Rapid heating and cooling rates are the key features that enable the formation of a very fine microstructure and which improves the hardness and the wear resistance of the laser-treated component.[5–8] For example,[9] the hardness increase associated with a martensitic transformation was observed in carbon steels after exposure to a pulsed laser. Compared to other conventional methods for selective surface hardening, the benefits of laser surface processing include fine grained and homogeneous microstructures, low thermal damage of the underlying substrate, reduced grain growth and distortion, formation of nonequilibrium and amorphous structures, and extension of E. COLOMBINI, Post Doc, R. SOLA, Assistant Professor, P. VERONESI, Associate Professor, and G. POLI, Full Professor, are with the Department of Engineering (DIEF), University of Modena and Reggio Emilia, Via Vignolese 905/A, 41125 Modena, Italy. Contact e-mail: [email protected] G. PARIGI, Technical Director, is with the Stav, Barberino del Mugello, Italy. Manuscript submitted February 18, 2014. Article published online August 6, 2014 5562—VOLUME 45A, NOVEMBER 2014
the solid solubility of alloying elements.[8] These benefits, in turn, enhance the tribological and mechanical properties, including hardness, strength, toughness, fatigue, wear, and corrosion resistance, while the bulk properties remain unchanged.[10] Parameter variations, such as exposure time, laser beam wavelength, temporal pulse power (pulse length, peak power, and pulse shape), repetition rate beam energy distribution, and beam geometry (focal spot size, depth of focus), may result in modified properties in the heat affected zone and transition zone.[1,7] In laser heating the energy from the laser beam can also be used to alter the properties of the surface by alloying with a second phase.[1,10,11] This method has the advantage of being highly suitable for treating a local volume of a part in place of the whole volume, thus reducing plastic deformations of the processed parts and providing a specified set of physical and mechanical properties by alloying with differe
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